This invention relates to a battery discharging apparatus, and in particular to an apparatus that recovers, by deep discharge, a battery's real power capacity which has been reduced due to the memory effect.
In general, phenomenon called memory effect is well known. Memory effect, wherein battery voltage drops in two stages during deep discharge after a repetition of shallow charge-discharge cycles, occurs in rechargeable batteries such as nickel-cadmium or nickel-hydrogen batteries. Turning to FIG. 1, the dashed curve, A, represents the discharge characteristics of a battery with memory effect, while the solid curve, B, represents the discharge characteristics of a normal battery without memory effect. This figure shows that as discharge progresses, voltage drops after a shorter service time for a battery with memory effect. Consequently, when a battery with memory effect is used in equipment that cuts off at a final voltage, the battery's real power capacity is reduced.
In this way, when memory effect occurs in a rechargeable battery used as a power source in various battery driven equipment, problems occur such as the working voltage of the rechargeable battery dropping below the operating voltage of the equipment, causing the operating equipment to suddenly cease operation. On the other hand, battery driven equipment such as the eight millimeter video camera electrically disconnect the battery and cease camera operation when the battery voltage drops below the camera's operating voltage. This is to avoid unpredictable equipment behavior caused by driving the equipment at a reduced battery voltage.
Often the operating voltage for this type of equipment is set at a value that does not completely discharge the battery and leaves it with some residual power. Therefore, memory effect becomes particularly likely in these applications where the battery's charge discharge cycle is repeated.
Moderately deep discharge techniques have been developed to eliminate the memory effect (see, for example, Japanese public disclosure 198516, 1987). The discharging apparatus described in this disclosure contains a discharging means to deeply discharge a rechargeable battery when its voltage drops below a set value. The discharging means utilizes either a timer to achieve deep discharge until a set time interval has passed or a battery voltage sensor to achieve deep discharge until a set voltage is reached.
The discharging apparatus described in this disclosure is characterized in that real battery power loss due to memory effect is recovered by deep battery discharge, or namely by memory effect eliminating discharge. Thus memory effect eliminating discharge is done to recover a battery's real power capacity when it has been reduced due to memory effect. For example, memory effect eliminating discharge would be accomplished when a battery's real power capacity dropped to 80% of its normal capacity. However, even deep discharge of a battery having no real power reduction due to memory effect produces no recovery of power capacity. Memory effect eliminating discharge of a battery with no memory effect not only wastefully consumes power, but also shortens the discharge cycle lifetime of the battery. Further, performing memory effect eliminating discharge every time battery voltage drops does not recover the battery's original real power capacity.
A discharging apparatus that automatically performs memory effect eliminating discharge whenever the battery voltage drops below a set value has the drawback that since memory effect eliminating discharge is accomplished every time the battery voltage drops, time is consumed deeply discharging tile battery as well as recharging it from the deeply discharged state.
The present invention was developed to eliminate these drawbacks. It is thus a primary object of this invention to provide a battery discharging apparatus that can eliminate memory effect by discharging the battery at appropriate times.